document.write limit

I've read in some books that there is a limit to how much text you can write to a document using the document.write() command. So I try not to use that command much. But I have no idea about the limit. Is it 100 characters? 1000 characters? 10000 characters? I assume it depends on the browser, too. Do recent browsers let you write more text that way?
How about Ajax? When you put text in a document with Ajax, is there a limit?